It is known from U.S. Pat. No. 4,285,730, U.K. patent application No. 2,069,994, and European Patent Application No. 0019342 that it is possible to obtain optical elements exhibiting precision surfaces, such as spherical or aspherical lenses, by direct molding or pressing without requiring polishing or other subsequent finishing operation, by using glasses having low transition temperatures Tg (corresponding to a viscosity of 10.sup.13 poises) and softening temperatures or Littleton points T.sub.L (corresponding to a viscosity of 10.sup.7.6 poises). The glasses are formed under pressure at a viscosity generally lying between the two values previously mentioned.
In order not to alter the surface of the molds, one conveniently operates at a molding temperature as low as possible, and preferably lower than 450.degree. C., which requires the glass to have a softening point below 500.degree. C.
Those optical components being destined for the fabrication of complex optical systems, performing and varying, can admittedly be more easily attained if the optical characteristics (refractive index and Abbe number or dispersion) available are numerous and different, as in the case in traditional optical glasses.
U.K. patent application No. 2,069,994 proposes moldable fluorophosphate glasses largely answering those needs. Nevertheless, that patent does not permit the production of glasses stable against devitrification, particularly in the fabrication of massive pieces, which exhibit properties within two regions of optical properties which can be attained only at the application limits of the patent, i.e., at high concentrations of either lead oxide or barium oxide or at a high level of alkaline earth oxides+lead oxide.
But, those two ranges are particularly important in the case of traditional glasses: these are, respectively, the region of "Extra Dense Flints", "Special Barium Flint", "Dense Barium Flint", having an index of refraction n.sub.D greater than 1.68 and an Abbe number v.sub.D less than 38, and the region of the "Dense Barium Crown" having an index of refraction n.sub.D greater than 1.57 and an Abbe number v.sub.D greater than about 55 and, preferably, higher than about 58.
Besides their thermoviscosity and optical properties, the glasses must exhibit good chemical resistance against atmospheric agents or "weathering".